Successful liquid sample introduction into gas-phase or particle detectors is dependent upon the interface between the liquid stream and the detector. The co-existence of continuous liquid sample introduction and normal operating requirements of the gas-phase detectors present compatability problems, Difficulties are sometimes encountered in accommodating the mass flow from the liquid stream into the detector. In addition, degradation of thermally labile sample components may occur during the evaporation processes prior to gas-phase detection. In the case of gas-phase detectors such as mass spectrometers, where detection occurs at a reduced pressures, vacuum locks and pumping requirements may be considerations. General requirements for interfaces between liquid streams and gas-phase detectors are: (1) The sample must be evaporated prior to detection; (2) Minimal thermal degradation should occur during the sample evaporation process; (3) The sample transport efficiency should be sufficiently high so that adequate sensitivity is observed; (4) The normal operating conditions of the detector should be maintained during sample introduction; and (5) The sample's composition should be maintained while being transported to and through the interface (e.g. minimal chromatographic band broadening). Success in interfacing liquid streams to gas-phase detectors depends on how well the foregoing requirements are met.
The principal application of the present device to gas-phase detection of liquid streams is the introduction of the effluent from a liquid chromatograph into a mass spectrometer. The interfacing between liquid chromatography (LC) and mass spectrometry (MS) is referred to as LC-MS. Although the present invention relates to the general field of liquid sample introduction into gas-phase or particle detectors, most prior work in this area has concentrated on LC-MS because it has presented formidable obstacles to interface design. This background discussion therefore focuses on LC-MS.
Complex gas-phase detectors, mass spectrometers, detect gas-phase ions formed by a variety of mechanisms; electron impact (EI) ionization and chemical ionization (CI) are the most commonly practiced approaches. With EI ionization, the sample gas at 10.sup.-3 -10.sup.-6 torr is bombarded with electrons of sufficient energy (generally 70 eV) to excite electronic energy levels of sample molecules beyond the ionization potential so that a electron is removed from the sample molecule, making it a positive ion. Upon sample ion formation in the EI mode, excess energy imparted to the sample molecule from the bombarding electron causes bond cleavage or fragmentation. It is a characteristic and reproducible nature of EI fragmentation, indicative of molecular structure, that provides this technique with broad utility for the analysis of samples with an unknown composition. In contrast, the CI mode operates at higher pressures relative to EI (typically at one torr), whereby the ionization occurs due to collision of sample molecules with reagent gas ions. The analytical utility of CI is generally found in the presence of molecular weight information. With CI, the limited and irreproducible fragmentation of sample molecules is essentially of no value. It should be stressed that the ionization process ultimately determines the qualitative information obtained with the mass spectrometer. Alternative ionization techniques are atmospheric pressure ionization (API) at 760 torr and field ionization at 10.sup.-4 torr. It is preferable to use a variety of ionization techniques, including EI and CI, to obtain the maximum information for a given sample. LC-MS techniques that place a restriction upon ionization conditions also are limited in the sample information obtained for a given analysis.
Mass spectra of many compounds, usually those ionized under EI conditions, have been compiled in massive computerized data-base libraries for subsequent comparison with acquired spectra (fragment ions) from samples of unknown composition. It would be therefore a significant advantage for LC-MS devices to make use of such spectral libraries because computer comparisons can be made in a matter of seconds. Hence, for the wide use there is a need for LC-MS devices that utilize EI ionization modes. Unfortunately, only few prior art devices are reported to have the capability of producing EI spectra of thermally labile and/or involatile compounds.
Effluent from either a liquid chromatograph or a liquid process stream must be accommodated by the mass spectrometer interfacing techniques. The pressure requirements for ionization, as discussed, are dependent upon the mode of ionization, and limited by the mass flow into the ionization region of the mass spectrometer and the pumping capacity of the mass spectrometer. The evaporation of liquid flowing at 1-2 mL/min may produce as much as a liter per minute of gaseous sample at STP (Standard Temperature and Pressure), which amounts to 10.sup.8 liters per minute of gas at 10.sup.-5 torr (i.e. EI conditions), far exceeding the pumping capabilities of conventional mass spectrometers. Ionization techniques which occur at pressures of one millitorr or higher, such as CI, require less pumping but usually result in significant ion-molecule reaction chemistry which yields little structural fragmentation information. Due to the requirement of low pressure for EI ionization, direct introduction of a continuous stream of liquid from a liquid chromatograph is difficult to attain without extremely large capacity pumping systems such as cryogenic pumping.
The evaporation or desorption process, whereby the sample is transformed into gas, may also result in thermal degradation (pyrolysis), reactions, or rearrangements of the original sample molecules. These sample losses are most prevalent with sample components that are thermally labile and/or involatile, commonly separated by liquid chromatography. Mass analysis of these thermally labile and/or involatile molecules is usually limited by the inability to produce intact gas-phase ions of these species. Therefore, it is important in development of interfaces between the LC and MS to vaporize the sample with minimal degradation or loss of analyte.
There have been a variety of approaches to interfacing the LC with the MS and this work has been extensively reviewed (1,2,3,4). The common objective of all interfacing techniques is efficiency in the production of gas-phase sample ions.
Direct Liquid Introduction (DLI) is one of the simplest approaches to interfacing LC with MS. With DLI, the effluent from a liquid chromatograph flows through a tiny circular aperture or tube with a diameter on the order of three to ten micrometers. A high velocity cylindrical liquid jet is directed into the ionization chamber. There have been a wide variety of designs using this approach and they all have the same basic configuration (reviewed in 5 and 6). The jet may proceed through a heated desolvation region before entering the ionization region to aid in solvent evaporation. This technique typically has been limited to micro-bore LC flow rates, less than one hundred microliters per minute. To accommodate direct introduction of liquid into ion source, cryogenic pumping has been used to trap the excess sample onto a cold surface of the outside of the ion source. In cases where normal LC flow rates are used, 1-2 mL/min, the effluent has been split, leaving only a fraction of the sample to be sampled into the mass spectrometer. Another limitation of the DLI technique is that the spectra produced yield only CI data. Little or no structural information is thus obtained, in contrast to EI. In addition, more costly differential pumping is required to maintain the mass analyzer pressures sufficiently low. In practice DLI has been plagued by repeated clogging of the micron sized orifices, causing the approach to be cumbersome, with significant downtime. The alignment and the instability of micron sized liquid jets also make DLI experimentally difficult in that data acquired may be noisy and irreproducible. However, the advantage of this technique is the lack of thermal degradation when analyzing thermally labile compounds. Further discussion of this technique may be found in U.S. Pat. No. 3,997,298 of Dec. 14, 1976 and U.S. Pat. No. 4,403,147 of Sept. 6, 1983.
Mechanical Transport (MT) is an LC-MS approach whereby the effluent from the LC is deposited on a moving surface, such as a wire or belt. Heat may be applied to the sample to remove solvent and the desolvated sample is mechanically transported on the wire or belt through a series of vacuum locks into the low pressure ion source of the mass spectrometer. Both EI and CI mass spectra have been obtained with this technique. A limitation of this technique is the requirement that the sample be evaporated or desorbed from the moving surface prior to ionization. Thermal degradation may occur during the thermal evaporation process. Operation of mechanical transport devices is frequently cumbersome due to design complexity and jamming of moving parts. The chromatographic profile of sample may be degraded by the non-uniform application of sample upon the moving surface. This approach is explained in greater depth in U.S. Pat. No. 4,055,987 of Nov. 1, 1977.
Thermospray (TSY) is a more widely used approach to LC-MS. The effluent from the LC flows through a thermal vaporizer into a heated vaporizer chamber in the ion source region of the MS. The thermal vaporizer transforms the sample into an ion-vapor plasma in a vaporizer chamber. A small fraction of the ion-vapor is sampled into the ion optics region of the mass spectrometer through a small aperture. The efficiency of sampling analyte through the sampling aperture is quite low. The majority of the ion-vapor is evacuated through a roughing line connected to the vaporizer chamber. As with DLI, costly differential pumping between the ion optics region and the mass analyzer region is required to maintain an adequate vacuum. The vaporization process produces gas-phase reagent ions when buffered solutions, such as aqueous ammonium acetate, are pumped through the thermal vaporizer. This ionization process, known as thermospray ionization, produces CI-like spectra. Under normal operational conditions, thermal degradation has been observed with the use of the thermal vaporizer; however, a large number of thermally labile compounds have been analyzed with this technique with minimal degradation. TSY has several limitations, most notably is the lack of structural information such as that obtained under EI ionization conditions. The response of various compounds depends upon the chemical nature of the substance being analysed. Consequently, it is sometimes difficult to predict response for poorly characterized samples. Thermospray processes are described in further detail in Canadian Patent No. 1,162,331 of Feb. 14, 1984, and U.S. application Ser. No. 527,751, filed Aug. 30, 1985, and a continuation thereof filed Ser. No. 832,743, issued as U.S. Pat. No. 4,730,111, to M. Vestal and C. Blakely on Mar. 8, 1988.
Monodispersed Aerosol Generation Interface for Combining liquid chromatography with mass spectrometry (MAGIC) is an approach to LC-MS whereby effluent is pumped through a tiny orifice or tube, forming a stable liquid jet. The liquid jet breaks up into uniformly sized or monodispersed droplets. The droplets are dispersed in a near-atmospheric pressure desolvation chamber with a dispersion gas that serves to prevent coagulation of the droplets as well as conduct thermal energy to the droplets resulting in rapid desolvation. This approach requires a large diameter desolvation chamber at near atmospheric pressure to allow efficient desolvation. The effect of lowering desolvation chamber pressure on the rate of solvent evaporation has theoretically been treated by Fuchs and Sutugen (16). The rate of evaporation of a liquid droplet is significantly reduced with decreases in pressure. In the absence of dispersion gas, the droplets receive insufficient thermal energy which prevents their complete desolvation. Dispersion gas flows of greater than one liter per minute have been used in order to maintain sufficiently high pressure in the desolvation chamber. Subsequent to desolvation, solvent-depleted solute particles are accelerated through a nozzle into a vacuum chamber to form a high velocity aerosol beam. The lighter solvent vapor and dispersion gas, compared to the more massive solute particles, expand outward from the axis of the aerosol beam, leaving a collimated particle beam devoid of gaseous components. The gaseous components of the aerosol beam are removed in a two-stage pressure reduction process, accomplished by directing the particle beam through two successive skimmers that separate two successively lower pressure vacuum chambers. The solute particle beam proceeds through the skimmers into the ion source region where enriched solute is thermally desorbed from surfaces in the ion source region and ionized by a conventional CI or EI ionization process.
The MAGIC approach to LC-MS has the advantage of ionizing solute under EI conditions. However, the requirement of near atmospheric pressure desolvation significantly reduces the solute transport efficiency into the low pressure ion source of the mass spectrometer. The addition of high flow rates of dispersion gas creates turbulence at the nozzles and significant loss in transport efficiency is observed due to impact on the surfaces of the skimmers and nozzles as well as walls of the desolvation chamber. The requisite high gas lead also tends to increase the solid angle expansion of the particle beam and to favor the use of a less efficient two-stage separator device. Because of these conditions, transport efficiency of solute into the ion source is generally on the order of five percent. With MAGIC, the mobile phase composition does not affect the response for various analytes as does the thermospray technique which in some cases requires mobile phase additives for sensitive response. Also, no differential pumping is required with this technique when EI ionization is the only mode employed. Additional details on this technique are presented in U.S. patent application Ser. No. 623,711 filed June 22, 1984, by R. Browner and R. Willoughby, which issued Dec. 16, 1986 as U.S. Pat. No. 4,629,478, and in a continuation thereof.
MAGIC can be considered a particle beam introduction technique. For this discussion, particle beam introduction is considered as a technique of accelerating an aerosol through a nozzle into successive vacuum chambers while skimming the aerosol particles on axis, forming a particle beam, and pumping away gaseous components of the aerosol beam off-axis. The result of this process is the efficient separation of aerosol particles from gaseous material, with the particles being transported more efficiently into lower pressure regions because of the higher momentum of the particle when compared to the gas molecules. Prior particle beam introduction techniques for mass spectrometry have applied to two areas: (1) Real-time aerosol monitoring (7-9); and (2) Liquid sample introduction where an aerosol generation step precedes the particle beam introduction (10-14). The MAGIC technique is an example of the latter. The present invention also includes a particle beam solute enrichment step when applied to sample introduction into the mass spectrometer.
Performance of particle beam introduction techniques is dependent upon the properties of the aerosol. The solid angle dispersion of the particle beam is dependent upon the size of the solute particles, the pressure from the aerosol source, and the geometry of the nozzle. Israel and Friedlander (15) experimentally showed the relationships of these parameters on particle beam dispersion. Their results show: (1) Particle beam angular dispersion increases with aerosol source pressure: (2) Particle beam angular dispersion decreases with increase in particle size: and (3) Particle beam expansion is more uniform with changes in particle size when capillary versus converging nozzles are used. Therefore, the nature of the aerosol generation process in terms of gas flow and pressure, and particle size and distributionultimately determines the efficiency of the particle beam introduction technique.
A variety of aerosol generators have been used with the particle beam approach to liquid sample introduction into the mass spectrometer. These include the Berglund-Liu monodisperse aerosol generator (8,10-14), the Willoughby-Browner monodisperse aerosol generator (14), and DeVilbiss and ultrasonic nebulizer (10-13). A major limitation of prior particle beam techniques was the difficulty in desolvation of the aerosol droplets subsequent to aerosol generation. Prior techniques required a desolvation chamber or increased gas load to remove solvent from the droplets. The generation of droplets greater than about ten microns in diameter with prior aerosol generation techniques leads to greater likelihood of particle losses in the desolvation chambers and nozzles due to impaction or settling process. The aerosol generation process of the present invention is designed to permit precise control over aerosol properties, including the droplet size, direction, and rate of evaporation. With enhanced control over the aerosol generation and desolvation processes, the efficiency of the particle transport to various detectors is increased.
The solid angle dispersion of particle beams has been shown to increase with increases in aerosol source pressure (15). Thus, the prior particle beam techniques that require high gas loads for aerosol generation or desolvation tend to have more divergent particle beams. This requires that only part of the particle beam cross-section can be sampled through axial skimmers because pressures in subsequent chambers exceed the upper pressure limitation of the detector. Thus, the entire cross-section of a less divergent particle beam could in theory be collected with the same skimmer diameter while maintaining the same detector pressure. The result of a less divergent particle beam is more efficient sample transport to the detector. Consequently, an objective of the present device is to decrease the gas load from the aerosol generation process to enhance sample transport efficiency.
The use of particle beam techniques for sample introduction into the mass spectrometer has demonstrated the ability to produce spectra under electron impact ionization conditions (7-14). A major objective of the present device is to enhance the ability to volatilize the particles once the particle beam enters the ion source region of the mass spectrometer. The objective is to form intact gas-phase molecular species of substances originating in the particle. But prior particle beam sample introduction devices have experienced difficulty in forming intact molecular ions due to thermal fragmentation of molecules during evaporation from heated surfaces (8). Particle volatilization process depends upon the equilibrium surface vapor pressure of molecules originating from the particles, the temperature and material of the particle beam collection surface, and the presence of other components in the particle matrix. Control of these factors is essential to the performance of the present device.
Other applications of liquid sample introduction into gas-phase or particle detectors have been reported for light scattering (17), flame ionization (18), atomic absorption or emission spectrophotometry (19). The enhanced control of aerosol generation, desolvation and solute enrichment with the present device is applicable to a variety of detectors.
Sources for the above mentioned in the above background history are:
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The above sources are incorporated by reference herein.